1. Field of the Invention
The present invention relates to an RFID (Radio_Frequency_IDentification) tag which exchanges information with external devices in a non-contact manner. Incidentally, the “RFID tag” described herein is also known as an “RFID tag inlay” among those skilled in the art, meaning a component laid into the RFID tag. Also, the “RFID tag” is sometimes called a “wireless IC tag.” RFID tags also include contactless IC cards.
2. Description of the Related Art
Recently, various types of RFID tag have been proposed to exchange information with external devices typified by reader-writers in a non-contact manner by radio. A configuration in which an antenna pattern for wireless communications and an IC chip are mounted on a base sheet made of plastics or paper has been proposed as a type of RFID tag (see For example, Japanese Patent Laid-Open Nos. 2000-311226, 2000-200332, and 2001-351082). One possible application for RFID tags of this type is to affix them to goods or the like and identify the goods by exchanging information about the goods with external devices.
FIG. 1 is a plan view showing an example of a RFID tag.
The RFID tag 1 shown in FIG. 1 consists of an antenna 12 installed on a sheet-like base 13 made of a PET film or the like and an IC chip 11 electrically connected to the antenna 12 by gold, solder, or the like and bonded to the base 13 with an adhesive.
The IC chip 11 of the RFID tag 1 can exchange information with external devices by conducting wireless communications with them via the antenna 12.
Although in FIG. 1, the antenna 12 of the RFID tag 1 is shown as a loop antenna, the antennas 12 for RFID tags in general are not limited to this shape and an antenna which extends linearly in both directions from an IC chip 11 placed at the center or antennas of various other shapes may be adopted.
RFID tags such as described above may deteriorate greatly in communications performance if there is a metal strip or the like nearby. A RFID tag called a metal tag is known as a means of avoiding such a situation. The metal tag is a RFID tag consisting of a substrate surrounded by a metal pattern which acts as an antenna. Even if a metal strip comes close to it, the metal tag maintains its communications performance except for the part shaded by the metal strip.
Now, a conventional manufacturing method of the metal tag will be described.
FIGS. 2(A) and 2(B) are perspective views of parts used in the manufacture of the metal tag.
An IC chip 11 (FIG. 2(A)) and a substrate 20 (FIG. 2(B)) for the metal tag are prepared here.
As shown in FIG. 2(A), the IC chip 11 has bumps 11a of gold or the like formed on its connection terminals. In FIG. 2(A), the IC chip 11 is shown upside down from the view of the IC chip 11 in FIG. 1 so that the surface on which the bumps 11a are formed will be visible. The IC chip 11 has capability to exchange information with external devices by conducting wireless communications with them via the antenna (described later) (see FIG. 1).
The substrate 20 consists of a dielectric plate 21 with an antenna metal pattern 22 going around the dielectric plate 21 excluding an area 23 on which the IC chip 11 will be mounted, where the antenna metal pattern 22 will act as an antenna after assembly.
FIGS. 3(A) to 3(C) illustrate a process chart showing an example of a manufacturing method for the metal tag.
Liquid or sheet-like underfill 24 that is a thermosetting adhesive is supplied to that area 23 of the substrate 20 on which the IC chip 11 will be mounted (FIG. 3(A)). The IC chip 11 is placed on the area 23 and then heated and pressurized together with the substrate 20 by being held between a heating stage 31 and a heating head 32. Consequently, the IC chip 11 and antenna metal pattern 22 are electrically connected via the bumps 11a and the IC chip 11 is fastened to the substrate 20 as the underfill 24 hardens (FIG. 3(B)).
Through these processes, the RFID tag with the structure shown in FIG. 3(C) is manufactured.
In the RFID tag, the IC chip 11 conducts wireless communications with external devices via a loop antenna which makes a round, surrounding the front and back surfaces of the dielectric plate 21.
The RFID tag of this type is known as a metal tag. For example, even if a metal strip is brought close to the back side of the substrate 20, the front side on which the IC chip 11 is mounted maintains sufficient communications performance.
However, the RFID tag produced by the manufacturing method described with reference to FIGS. 2(A)-2(B) and 3(A)-3(C) is difficult to make flat and thin because the IC chip 11 mounted on the substrate 20 protrudes from the surface of the substrate 20. To solve this problem, it is at least conceivable to reduce the thickness of the substrate 20. However, in order to obtain desired performance as a loop antenna from the antenna metal pattern 22, it is necessary to provide some distance between those parts of the antenna metal pattern 22 which are located on the front surface and back surface of the substrate. Thus, there is a limit to reducing the thickness of the substrate 20 from the viewpoint of ensuring antenna performance.